Of all animals possessing a backbone, human beings are the only creatures who remain upright for significant periods of time. From an evolutionary standpoint, this erect posture has conferred a number of strategic benefits, not at least of which is freeing the upper limbs for purposes other than locomotion. From an anthropologic standpoint, it is also evident that this unique evolutionary adaption is a relatively recent change, and as such has not benefited from natural selection as much as have backbones held in a horizontal attitude. As a result, the stresses acting upon the human backbone (or “vertebral column”), are unique in many senses, and result in a variety of problems or disease states that are peculiar to the human species.
The human vertebral column is essentially a tower of bones held upright by fibrous bands called ligaments and contractile elements called muscles. There are seven bones in the neck or cervical region, twelve in the chest or thoracic region, and five in the low back or lumbar region. There are also five bones in the pelvis or sacral region which are normally fused together and form the back part of the pelvis. This column of bones is critical for protecting the delicate spinal cord and nerves and for providing structural support for the entire body.
Between the vertebral bones themselves exist soft tissue structures, or discs, composed of fibrous tissue and cartilage which are compressible and act as shock absorbers for sudden downward forces on the upright column. The discs allow the bones to move independently of each other, as well. The repetitive forces which act on these intervertebral discs during repetitive day-to-day activities of bending, lifting, and twisting cause them to break down or degenerate over time.
Presumably because of humans' upright posture, their intervertebral discs have a high propensity to degenerate. Overt trauma, or covert trauma occurring in the course of repetitive activities disproportionately affect the more highly mobile areas of the spine. Disruption of a disc's internal architecture leads to bulging, herniation, or protrusion of pieces of disc and eventual disc space collapse. Resulting mechanical and even chemical irritation of surrounding neural elements (spinal cord and nerves) cause pain, attended by varying degrees of disability. In addition, loss of disc height relaxes tension on the longitudinal spinal ligaments thereby contributing to varying degrees of spinal degenerative instability such as spinal curvature.
The time-honored method of addressing the issues of neural irritation and instability resulting from severe disc damage have largely focused on removal of the damaged disc and fusing the adjacent vertebral elements together. Removal of the disc relieves the mechanical and chemical irritation of neural elements, while osseous union (bone knitting) solves the problem of instability.
While interbody and instrumented posterolateral fusions are well known in the art, and frequently performed, one recurring problem accompanying these procedures is the accurate placement of the bone graft to ensure a long term stable arthrodesis. Hardware placement alone will not suffice as metal fatigue will eventually result in the breakage, loosening, or subsidence of virtually any spinal hardware not supported by a bone fusion mass.
While it is relatively easy to place bone graft material in open spinal fusion procedures, it is much more difficult to place adequate bone graft in situations where minimally invasive techniques are employed as the access channel to the spine is generally not much larger than that required to place the hardware percutaneously.
Percutaneous placement of pedicle screws is frequently employed either as a primary method of utilizing stabilizing hardware or as a back up to an interbody fusion. While it is relatively easy to place pedicle screws percutaneously under fluoroscopic guidance, and pass a connecting rod between pedicle screws, it is much more difficult to decorticate the laterally placed transverse processes and place bridging bone graft along them without enlarging the percutaneous incisions substantially or essentially converting the operation to a de facto open Wiltse approach, which requires a sizable incision and defeats the purpose of percutaneously placed minimally invasive pedicle screws.
Thus, there is a long felt need for a device that allows placement of adequate bone graft prior to or after a minimally invasive instrumented spinal fusion such that a stable long term arthrodesis can occur, wherein conversion to an open approach is not necessary simply to place adequate bone graft material.